1. Field of the Invention
The present invention relates to a polymer optical waveguide pattern formation method using a polymer material. The present invention can be utilized in various optical waveguides, optical integrated circuits, optical wiring boards and the like which are used in general optical and micro-optical areas and in optical communication or optical information processing areas.
2. Description of the Related Art
Pushed by market requirements and national policy, construction of high-capacity optical fiber networks and preparation of FTTX (fiber to the X point) are being promoted. That is, WDM-MUX/DEMUX (Wavelength Division Multiplexing-Multiplexer/ Demultiplexer) using an arrayed waveguide grating (AWG) as a key device has reached a practically applicable level, and a high-capacity and high-expandability network has become available. The demands of the market are expected for changes into optical networks of large-scale nodes, local networks, and various LAN systems in addition to transmission lines and MUX/DEMUX, in the future.
Polymer material is an optically isotropic amorphous material of which optical propagation loss is low as is inorganic glass. Application of polymer material to passive optical circuits is expected to be promising. Further, utilizing its thermo-optic (TO) constant, which is an order of magnitude greater than glass, polymer material has begun to be employed as a waveguide material to fabricate TO switches and the like. Specific waveguide materials can include acrylic polymer, acrylic resin, polyimide, silicone resin, epoxy resin, polycarbonate and the like. Various characteristics are required for waveguide materials. Among them, transparency, heat stability, optical isotropy, and processability are particularly important characteristics.
Most polymer materials are highly transparent in the visible region. On the other hand, overtone of vibration absorption of carbon-hydrogen bond (such as hydrocarbon skeleton) or oxygen-hydrogen bond (such as hydroxy group) causes decrease in transparency in the near infrared region, which is considered as communication wavelength region. Therefore, a fluorocarbonization of the basic skeleton and introduction of siloxane skeleton are being attempted.
A rigid polyimide skeleton, resilient siloxane skeleton, and bridged structure formed by heat or light are being employed to improve heat stability.
A component having optical anisotropy such as aromatic ring should not be oriented in order to improve optical isotropy. However, heat stability and optical isotropy are difficult to realize simultaneously because the rigid or resilient skeleton to improve the heat stability as described above promotes orientation of molecule.
When optical waveguides are fabricated, the processability primarily indicates formability of core-clad layer structure. When a high molecular-weight polymer material is spin-coated from a solution, an intermixing between core and clad layer tends to occur, which is often a problem in waveguide processability. On the other hand, when a low molecular-weight oligomer is spin-coated and then bridged by light or heat, since the bridged polymer film becomes insoluble in a solvent, intermixing can be prevented. As a result, it tends to have a superior processability.
Polymer materials are suitable for producing large-area optical waveguides because they are readily formed into thin films by a spin coating method or a dipping method. Further, according to such a method, since film is not formed at high temperatures, it has an advantage that optical waveguides can be constructed on substrates such as semiconductor substrates or plastic substrates which are difficult to be heat-treated at high temperatures. Still further, it is possible to produce a flexible optical waveguide utilizing the flexibility or tenacity of polymer materials. For such reasons, it is expected to produce optical waveguide parts in large quantity and at low cost by using polymer optical materials: such optical waveguide parts include optical integrated circuits used in the field of optical communications, optical wiring boards used in the field of optical information processing and the like.
Polymer optical materials have been considered to have problems in terms of environmental resistance such as heat stability or moisture resistance. However, a material with heat stability by introducing an aromatic group such as benzene ring and/or an inorganic polymer is disclosed recently, for example, in Japanese Patent Application Laid-open No. 3-43423(1991). Polymer materials have advantageous characteristics in thin film formation and heat treatment process as described above, and problems such as in heat stability or moisture resistance are being improved.
The following methods are reported to form polymer optical waveguides, such as a photo-locking or selective photo-polymerization method (Kurokawa et al., Applied Optics vol. 17, p. 646, 1978) in which a monomer is included in a polymer material, the polymer material is reacted partly with the monomer by irradiation with light to produce a refractive index difference between the irradiated part and unirradiated part; an applied method such as lithography or etching used in semiconductor processing (Imamura et al., Electronics Letters, vol. 27, p. 1342, 1991); and a method using a photosensitive polymer or resist which is superior in simplicity and mass production adaptability (Trewhella et al., SPIE, vol. 1177, p. 379, 1989). Further, a waveguide production method in which a photopolymerization initiator is added to an epoxy oligomer or the like, and a core is formed by irradiation of light, and then an uncured part is removed, is disclosed in Japanese Patent Application Laid-open No. 10-253845(1998).
As described above, there are many requirements for polymer materials used for optical waveguides. Among them, there are some requirements such as heat stability and optical isotropy. They are based on ideas contrary to each other in the molecular design. Consequently, there is very few material which meets all of such requirements at the same time. However, such a material is not absolutely unavailable, and there is an example of a thermosetting silicone resin: the compatibility between transparency and heat stability can be established by using a ladder siloxane skeleton; optical isotropy can be secured by random thermal crosslinking; and core-clad layer structure formation can be facilitated by thermally crosslinking a film formed with an oligomer and making the film insoluble in solvents.
As described above, silicone resin has superior characteristics as an optical waveguide material, but processability has not been satisfactory. For example, dry etching in the production of a core ridge requires a long time and a plurality of processes, like an inorganic material such as glass or semiconductor. Therefore, as to silicone resin for optical waveguide, as already realized in certain polymers, that is, a photocurable resin, it is desirable that the core-ridge can be directly produced by a simple method in which the resin is photo-crosslinked and an unreacted part is washed out by a solvent.
Normally, as a method for providing a silicone oligomer with a photocurability, a method is used in which epoxy group or vinyl ether group or radical polymerizable acrylic group is introduced into the silicone oligomer itself by covalent bond. However, in these methods, bond between side chains by crosslinking is dominant rather than siloxane bond, which generates problems not only in heat stability but also in an inevitable increase in optical propagation loss due to increase in ratio of hydroxy group in the case of epoxy or due to increase in ratio of hydrocarbon in the case of vinyl ether or acrylics.